1. Field of the Invention
The present invention relates a resonant element and a high frequency filter, and a wireless communication apparatus equipped with such a resonant element or high frequency filter.
2. Description of Related Art
Conventionally, wireless communication apparatuses that can transmit and receive wireless communication signals have been known. The radio frequency used in the wireless communication apparatus varies depending on the generation of the mobile communication system and on the region (e.g. country). For example, when it is desired to use both the second and third generation mobile communication systems through one wireless communication apparatus, or when it is desired to use a wireless communication apparatus in a plurality of regions, it is necessary to prepare a plurality of frequency bands that can be set and to switch to a desired frequency band to be used.
Therefore, in conventional wireless communication apparatuses, for example, a resonant element or a high frequency filter that has been tuned to a single frequency is prepared and provided for each of the frequency bands that are desired to be set, and the resonant element or the high frequency filter to be used is to be selected by a switch.
For example, there has been developed an antenna sharing device for allowing a transmitter and a receiver to share a single antenna. In order to achieve high attenuation and low loss without increasing the size of the antenna sharing device, the antenna sharing device is provided with five resonators, five capacitors and five switches etc, and the transmission band and the reception band can be switched in synchronized manner by turning on/off the switches. Specifically, there has been developed an antenna sharing device in which low bands (transmission low band and reception low band) and high bands (transmission high band and reception high band) can be switched over by turning on/off the switches (see, for example, Japanese Patent Application Laid-Open No. 11-243304).
As a further example, there has been developed a wide band transmission/reception apparatus. In order to enable an amplifier to perform amplification over a wide band by adjusting the resonance frequencies of resonance circuits, the apparatus is provided with two resonance circuits (each having an inductor and a capacitor connected in parallel with the inductor and further having four capacitors and four switches to switch over the parallel capacitances etc.) that enable switching between sixteen resonance frequencies by controlling switching of the switches by a 4-bit control signal, one of the resonance circuits being connected to the input of the amplifier and the other resonance circuit being connected to the output of the amplifier, wherein a desired frequency can be extracted from among frequencies of the signal to be received or transmitted by means of one resonance circuit, and the resonance frequency can be adjusted in accordance with the frequency of the signal to be received or transmitted by means of the other resonance circuit (see, for example, Japanese Patent Application Laid-Open No. 2006-325163).
As a still further example, there has been developed an antenna device or the like to be used in a portable terminal. In order to enable switching between frequencies over a wide band, the antenna device is provided with two separated antenna elements having first and second resonance frequencies, two power supply units that supply power to the two separated antenna elements respectively, and four switches, and switching between six resonance frequencies can be achieved by turning on/off the switches (see, for example, Japanese Patent Application Laid-Open No. 2006-086630).
As a still further example, there has been developed a flat antenna in which in order to enable transmission and reception of communication signals over a wide band with a single, small-size flat antenna, a plurality of slit-like cuts are formed on a radiating conductor and four switches each having contacts on the radiating conductors in such a way as to bridge each cut are provided, wherein switching between sixteen resonance frequencies is achieved by turning on/off the switches (see, for example, Japanese Patent Application Laid-Open No. 08-242118).
As a still further example, there has been developed a roadside antenna apparatus used in an ETC (Electronic Toll Collection) system or the like in which in order to adjust the phase of signals to be transmitted and received, a phase changing circuit is provided with three transmission paths for changing the phase values and six switches, wherein switching between eight phase values can be achieved by turning on/off the switches (see, for example, Japanese Patent Application Laid-Open No. 2003-023383).
In the conventional wireless communication apparatuses, however, an increase in the number of frequency bands desired to be set necessitates an increase in the number of resonant elements or high frequency filters and an increase in the number of switches too. Thus, the problem of cost incurred by the increase in the size of the circuit and an increase in the number of parts will be encountered.
In addition, an increase in the number of switches also leads to a problem of an increase in the signal loss.
In the case of the configuration disclosed in Japanese Patent Application Laid-Open No. 11-243304, it is necessary to provide capacitors etc. in addition to resonators and switches. In addition, the resonance frequency cannot be switched between more than two frequencies (two modes) though as much as five switches are used.
In the case of the configuration disclosed in Japanese Patent Application Laid-Open No. 2006-325163, it is necessary to provide two resonance circuits each having an inductor, a capacitor connected in parallel with the inductor, additional four capacitors and four switches to switch the parallel capacitances. In addition, the resonance frequency cannot be switched between more than sixteen frequencies though eight switches in total are provided for signals to be received (or signals to be transmitted).
Thus, neither Japanese Patent Application Laid-Open No. 11-243304 nor Japanese Patent Application Laid-Open No. 2006-325163 can give a solution to the problem of cost incurred by an increase in the size of the circuit and an increase in the number of parts encountered with an increase in the number of frequency bands desired to be set or a solution to the problem of increase in the signal loss caused by an increase in the number of switches.
Japanese Patent Application Laid-Open No. 2006-086630 and Japanese Patent Application Laid-Open No. 08-242118 pertain to switching of the resonance frequency of the antenna, where an antenna element or a radiating conductor is essential. Therefore, switching of the resonance frequency of a resonant element or a high frequency filter that does not have an antenna element or a radiating conductor cannot be achieved.
Although Japanese Patent Application Laid-Open No. 2003-023383 discloses a simple configuration including transmission paths and switches, it teaches switching of the phase value instead of switching of the resonance frequency. In addition, the phase value cannot be switched between more than eight values though as much as six switches are used.